1. Field of the Invention
The present invention relates to a transmission path selecting apparatus for mobile communications systems, to perform so-called hand-off switching control for transmission paths.
2. Description of the Related Art
In mobile communications, since the communication path it uses involves wireless sections, how to overcome timing lags in the transmission and reception of signals and the deterioration of signal transmission performance poses a major challenge.
Regarding the problem of how to eliminate timing lags in transmission and reception, the Gazette of the Japanese Patent Laid-open No. Hei 6-224875, for example, discloses a circuit which is illustrated in FIG. 5. This circuit is used to reduce jitters on the receiving side which arise when, in TDMA mobile communications accomplished via a base station, the transmission/reception timing is synchronized with the timing of the other party station to burst transmission/reception or of a higher-rank network.
The circuit illustrated in FIG. 5 consists of a clock generator 101, a phase comparator 102, a temporary memory 103, a frequency division phase controller 104, a frequency divicer 105 and a transmission/reception timing generator 106. Here the clock generator 101 supplies a clock NfCLK, which is N times a processing clock matching the transmission/reception timing. The frequency division phase controller 104 supplies a frequency division phase clock .phi. resulting from the addition, with N as modulus, of a phase offset obtained by accumulating the reference phase, which results from the frequency division of this clock NfCLK, and the phase difference output. The transmission/reception timing generator 106 supplies a desired transmission/reception timing output T0 and a gate timing signal GT. The phase comparator 102 detects a phase difference .epsilon. between a reception frame timing FT and a frame clock FC resulting from the frequency division of the frequency division phase clock .phi. by the frequency divider 105, and the temporary memory 103 stores this phase difference .epsilon.. Then, in this circuit, a frequency division phase clock .phi. matching the phase difference .epsilon. is selected when the gate timing signal GT is on, and this operation serves to absorb jitters.
On the other hand, addressing the deterioration of signal transmission performance, the Gazette of the Japanese Patent Laid-open No. Hei 1-274524 discloses a cellular telephone system using the so-called hand-off technique. The hand-off technique in this context means a control method whereby, when the state of communications during communication by one mobile station with another party via a radio base station, is deteriorated by fading or the like, the base station switches the radio base station connected to that mobile station to another and thereby keeps the state of communications satisfactory. In the system disclosed in the above-cited gazette, a data communications call is subjected to hand-off control on the basis of the signal strength, bit error rate, packet volume of radio channel data, traffic of data packets and throughput of data packets among other factors.
Incidentally, when the aforementioned hand-off control is carried out, because a lag may arise in the timing at which data packet signals are handled at the base station responsible for the transmission and reception of signals to and from the mobile station via radio base stations, inviting skipping of some data and/or other failures. To address this problem, though it is conceivable to apply the aforementioned technique to absorb jitters on the part of the base station, the jitters will be too great to be absorbed if the hand-off control invites a long timing lag, and some data will then be skipped.
The present invention, undertaken in view of the above-described circumstances, is intended to provide a transmission path selecting apparatus for mobile communications systems free from data skipping and other kinds of timing trouble even where hand-off control is carried out.